Azetidine-2-carboxylic acid is an unusual amino acid, the (S)-enantiomer of which is known to be useful in the synthesis of inter alia high molecular weight polypeptides, and in particular as an analogue of the well known amino acid proline.
This amino acid is of limited availability from natural sources, and in nature is found only as the (S)-enantiomer. The development of an efficient and economic synthetic method for producing both the pure racemic compound and either of the individual (R)- or (S)-single enantiomers is therefore desirable.
Previously documented chiral syntheses of (S)-azetidine-2-carboxylic acid include a five step preparation via homoserine lactone, starting from N-tosyl protected L-methionine (see for example Japanese Patent Application N.sup.o 14457/74 and Bull. Chem. Soc. Jpn. (1973) 46, 699) and a five step preparation via L-4-amino-2-chlorobutyric acid, starting from L-2,4-diaminobutyric acid (see Biochem. J. (1956) 64, 323).
Previously documented preparations of enantiomerically-pure azetidine-2-carboxylic acid from the racemate involve long and relatively complicated multi-step methodology.
For example, a four step preparation involving the protection, resolution and subsequent deprotection of racemic azetidine-2-carboxylic acid is known from J. Heterocyclic Chem. (1969) 6, 993. In this method, N-carbobenzyloxy-protected racemic azetidine-2-carboxylic acid is resolved using L-tyrosine hydrazide as resolution agent, and then isolated before a final deprotection step. This process has the further disadvantage that L-tyrosine hydrazide is expensive.
Such methods present the problem that they are typically cumbersome because of the need to recycle the resolving agent and inevitably only produce half of the material as the required isomer. For an economic overall process it thus becomes necessary to find a method for the recycling of the unwanted isomer, and for this to be integrated into a process with the minimum of extra chemical steps.
Moreover, racemic azetidine-2-carboxylic acid obtained via chemical synthesis inevitably contains contaminants. Thus a resolution procedure which produces only the required single enantiomer, as well as being more economic, is also expected to facilitate chemical purification of the product.
Bioresolution is a procedure which is known to be of use generally in the production of enantiomerically-pure compounds. However, the potential utility and effectiveness of the technique in the resolution of a particular chiral compound is difficult to predict.
No biocatalytic resolution method has been previously disclosed for azetidine-2-carboxylic acid. Moreover, no resolution has been disclosed for azetidine-2-carboxylic acid which integrates recycling of the unwanted isomer in an efficient manner and which takes account of impurities arising from the racemate synthesis.
We have now surprisingly found that enantiomerically enriched azetidine-2-carboxylic acid may be obtained in an extremely enantiomerically-pure form, and in extremely high yields, via a novel and efficient bioresolution process.